1. Field of the Invention
The present invention relates to a liquid crystal display panel in which an alignment film is located on a counter electrode supported by a support base member of a nearly rectangular shape and the rubbing direction of the alignment film is parallel to one side of the support base member.
2. Description of the Related Art
Liquid crystal display panels have been extensively used in personal computers, car navigation systems, TV monitors, etc., to display images. Liquid crystal (LCD) modes of the liquid crystal display panels generally include the TN mode and the STN mode, which utilize nematic liquid crystals. Further, an LCD mode is also known which uses ferroelectric liquid crystals or the like and which is high in response speed and wide in viewing angle; however, improvements in the resistance to mechanical shock, temperature characteristics and so on are required. In contrast to the LCD modes in which liquid crystal molecules are twist aligned, the optically compensated bend (OCB) mode is an LCD mode in which liquid crystal molecules are aligned parallel. This mode has attracted attention as being suitable for video equipment for displaying moving images because it is excellent in high-speed response and wide in viewing angle. For this reason, the OCB-mode liquid crystal display panels have been developed actively.
As with the other modes, the OCB-mode liquid crystal display panel has a structure such that a liquid crystal layer is held between an array substrate and a counter substrate which are a pair of electrode substrates. An alignment film is provided on each of the counter substrate and the array substrate in order to control the alignment of OCB liquid crystal molecules.
FIG. 15 shows a rubbing treatment which is performed on the alignment film AL of the counter substrate CT in the manufacturing process of an OCB-mode liquid crystal display panel. In the counter substrate CT, the alignment film AL is located on the counter electrode CE, which is made of indium tin oxide (ITO) or the like and supported by a support base member GL of a nearly rectangular shape. The edges of the counter electrode CE are inside those of the support base member GL and the edges of the alignment film AL are inside those of the counter electrode CE. In the rubbing treatment, a stage SG moves in the X direction together with the counter substrate CT placed on it as shown in FIG. 15. A rubbing roller RL is set in a slanting direction crossing the X direction at an angle and rotates around its axis together with a rubbing cloth BF wound onto it. The counter substrate CT is placed on the stage SG such that the two long sides of the support base member GL are oriented parallel to the rubbing roller RL and the two short sides are oriented perpendicular to the roller RL. The rubbing roller RL performs rubbing on the alignment film AL while the counter substrate CT moves under the rubbing roller RL. Here, the rubbing cloth BF rubs the alignment film AL in the Y direction perpendicular to the rotation axis of the rubbing roller RL in a state where it is in contact with the alignment film AL. That is, the rubbing direction of the alignment film AL is parallel to the short sides of the support base member GL. With the counter electrode CT and rubbing roller RL slanted as described above, the pile of the rubbing cloth BF is not irregularly deformed and makes the rubbing uniform. If the rubbing roller RL were perpendicular to the direction of movement of the counter substrate CT (the X direction), the pile of the rubbing cloth BF would be deformed so as to be divided to both sides, producing a valley in the rubbing cloth BF. The alignment film AL would not be sufficiently rubbed in this valley portion, thus causing nonuniformity in rubbing. To attain uniform rubbing, therefore, the rubbing roller RL is slanted not to form a right angle with the direction of movement of the counter electrode CT. Such a rubbing treatment is likewise performed on the alignment film of the array substrate as well.
Incidentally, the uniformity in rubbing significantly affects the display quality of the liquid crystal display panel. Nonuniform rubbing would cause nonuniformity in display in display operations. For example, the direction of movement of the counter substrate CT relative to the rubbing direction, the movement speed of the counter substrate CT and the rotational speed of the rubbing roller RL are factors which determine the uniformity in rubbing.
For example, Japanese Unexamined Patent Publication No. 10-186364 (patent document 1) and Japanese Unexamined Patent Publication No. 10-268311 (patent document 2) disclose techniques to solve the nonuniformity in display. The patent document 1 describes imposing restrictions on the direction in which piles are arranged and the angle of each electrode on the substrate relative to the rotating direction of the rubbing roller at rubbing time in order to solve the nonuniformity in rubbing resulting from the behavior of the pile of the rubbing cloth of the rubbing roller. The patent document 2 describes adjusting the relationship between the rotational speed of the rubbing roller and the moving speed of the substrate at rubbing time in order to solve the nonuniformity in rubbing due to the eccentricity of the rubbing roller.
However, the patent documents 1 and 2 do not take into consideration problems unique to the OCB mode caused by the rubbing treatment. In the TN and STN modes, only low pretilt angles are required. In contrast, in the OCB mode, high pretilt angles are required to stably transfer liquid crystal molecules from a splay alignment to a bend alignment. This is because the bend alignment becomes unstable if the pretilt is low.
In adjusting the alignment of liquid crystal molecules in the rubbing treatment, it is inevitable that the alignment state scatters (varies) to some degree, depending on the manufacturing process. In the OCB mode, since the pretilt is high, the influence of the manufacturing process appears as a much larger difference in pretilt angle than in the TN and STN modes. This large difference in pretilt angle forms the main factor of display nonuniformity on the display screen. If the nonuniformity in pretilt angle were large in the OCB mode, a change in intermediate-gradation display voltage would cause a large change in transmittance, resulting in remarkable nonuniformity in display. Thus, in a mode in which high pretilt is required, like the OCB mode, high uniformity is required.
The inventors considered variations in pretilt angle which cause remarkable display nonuniformity in the OCB mode. When the rubbing cloth BF of the rubbing roller RL shown in FIG. 15 comes into contact with one edge of the counter electrode CE which extends in the Y direction, the pile of the rubbing cloth BF suffers damage with the electrode edge. In general, the counter electrode CE is made of ITO. The ITO edge has an acute step. Rubbing parallel to this step causes the pile of a portion of the rubbing cloth BF to abrade and break and causes slices of the pile to stick to other peripheral portions in the rubbing cloth BF. The area thus damaged forms a spiral trace of damage as the counter substrate CT moves and the rubbing roller RL rotates. When rubbed by such a rubbing cloth BF, that portion of the alignment film AL which faces the damaged area will not be rubbed sufficiently. Thereby, stripe-like defects DF resulting in shortage of rubbing will be formed at regular intervals on the alignment film Al. The trace of damage cannot be ignored because it accumulates with displacement each time the rubbing treatment is repeated. In FIG. 15, the stripe-like defects DF form a nearly triangular area on the alignment film AL of the counter substrate CT passed under the rubbing roller RL. The nearly triangular area is nonuniform in the rubbing state, which results in variations in the alignment state of liquid crystals in the liquid crystal layer, i.e., variations in pretilt angle. Although the stripe-like defects DF cannot be confirmed by observing the alignment film AL, they are observed as display nonuniformity on image display after the completion of assembly of the liquid crystal display panel.
When the pile of the rubbing cloth BF rubs the edge of the alignment film AL adjacent to the edge of the counter electrode CE, a portion of the alignment film AL may peel off to get into the rubbing cloth BF as a particle in a position PT shown in FIG. 16. In that case, the particle would further damage the rubbing cloth BF. In addition, the particle would damage the surface of the alignment film AL facing the rubbing cloth BF each time the rubbing roller RL makes one rotation. For example, if the rotational axis of the rubbing roller RL is at 30° relative to the direction perpendicular to the direction of movement of the counter substrate CT (the X direction), the rubbing roller RL makes one rotation in 0.1 seconds, and the counter substrate CT moves 2 mm in 0.1 seconds, then stripe-like defects DF due to damage will be formed at a pitch of 1 mm on the alignment film AL as shown in FIG. 16.